Neonatal estrogen treatment of mice results in an increase in mammary gland tumorigenesis and in an increase in mammary gland sensitivity to subsequent exposure to a chemical carcinogen. This proposal will examine the mechanisms mediating the effects of early hormone exposure on mammary gland tumorigenesis. Specifically, this proposal will determine whether the effects of neonatal estrogen administration are due to direct action of the compounds on the mammary gland, to changes in systemic factors, or to a combination of mechanisms. This proposal will also examine changes in mammary gland prolactin, estrogen, and progestin receptor concentrations and affinities as possible mechanism by which neonatal hormone administration effects an increase in mammary gland tumorigenesis. Receptor concentrations and affinities will be correlated with tumor incidence, type, and age of appearance. Finally, this proposal will determine the age of administration at which DMBA is most effective in inducing mammary gland tumors. The increased sensitivity of the mammary gland that follows neonatal estrogen exposure will be correleated with changes in the growth rate of the gland that result from neonatal hormone treatment.